* refactor(core): serialize response once closes#5641
This change impacts both the custom protocol and the postMessage based IPC implementations. Basically it changes the whole IPC mechanism to work on raw JSON strings so we do not need to serialize a serde_json::Value after serializing to it from a user-provided type.
i benchmarked this with a 150MB file response (returning Vec<u8> instead of tauri::ipc::Response since the latter does not serialize at all) and it went from 29s to 23s (custom protocol) and from 54s to 48s (post message) on macOS.
* fix mobile & lint
* clippy
